The present invention relates to a Passive Optical Network (PON) system in which plural subscriber connection apparatuses share an optical transmission line, and an optical line terminal (OLT). More particularly, the present invention relates to a PON system and an OLT, in which the inclination of burst reception characteristics is moderate.
A PON system includes an OLT and plural Optical Network Units (ONUs). Each ONU converts an electrical signal from a terminal (such as PC) connected to the ONU into an optical signal. The individual ONUs transmit optical signals to the OLT through their subscriber optical fibers and an optical splitter. At this time, the optical signals are time-division multiplexed on the trunk fiber to the OLT. The OLT provides communication between a terminal of an ONU, and a terminal of a different ONU or a terminal of a different network (NW).
As specified in Sections 8 and 9 of ITU-T Recommendation G984.1, each ONU is located in one of the following three ranges: 0 to 20 km, 20 km to 40 km, and 40 km to 60 km in length of the optical fiber. However, the transmission distances of the nearest ONU and the farthest ONU differ as much as 20 km, and the transmission delays differ therebetween, so that the optical signals output from the ONUs may collide and interfere with each other. For this reason, the delays of the output signals from the ONUs are adjusted as if all the ONUs are located at an equal distance (such as 20 km) by the ranging technology specified in Section 10 of ITU-T Recommendation G.984.3. As a result, the optical signals from the ONUs do not interfere on the trunk fiber. Incidentally, the PON system can adjust the interference, but cannot adjust the attenuation due to the difference of the length of the optical fiber.
Further, as specified in Section 8.3.3 of ITU-T Recommendation G.984.2, a guard time, a preamble, and a delimiter are added to the head of the signal from the ONU. The guard time has 12 bytes and serves as a protection against the interference. The preamble is used for determination of the identification threshold of a receiver, as well as for clock extraction. The demiliter is used for identifying the boundary of the received signal.
In Section 8.2 of ITU-T Recommendation G.984.3, signals transmitted from plural ONUs to an OLT are referred to as upstream signals. The upstream signal includes a preamble, a delimiter, and a payload signal. Further, as shown in FIG. 8-2 of Section 8 of the recommendation, a guard time is provided immediately before each upstream signal in order to avoid collision with the last bust signal.
On the other hand, according to Section 8.1 of the recommendation, signals transmitted from the OLT to the plural ONUs are referred to as downstream signals. The downstream signal includes a frame synchronization pattern, a PLOAM field, a US Bandwidth MAP field, and a frame payload. As shown in Section 8.1.3.6 of the recommendation, the OLT specifies the timing of the upstream transmission permission for each ONU by use of the field called US Bandwidth MAP. The US Bandwidth MAP field includes a start value for specifying the start of the transmission permission, and an end value for specifying the end of the transmission permission, respectively on a per-byte basis. The values are also referred to as grant values, meaning that the transmission is permitted. The difference between the end value and the next start value is an upstream no-signal field corresponding to the guard time. Incidentally, it is possible to allocate plural bandwidth allocation units called T-CONTs to each ONU. The upstream transmission permission timing is specified for each T-CONT.
Ranging is performed in such a way that the OLT requests the ONU to transmit a distance measurement signal. The ONU returns a distance measurement frame to the OLT. Upon receiving the signal, the OLT measures the time period from the transmission request of the distance measurement signal to the reception of the distance measurement signal, namely, the OLT measures the round-trip delay time to discover how far the ONU is from the OLT. Next, the OLT transmits a message to each ONU to delay its transmission by a time called equalization delay so that all the ONUs appear to be located at an equal distance from the OLT. For example, the OLT specifies, for each ONU, an equalization delay that is equal to “(20 km round-trip delay time)-(measured round-trip delay time)” so that all the ONUs have the 20 km round-trip delay time. The ONU has a circuit for transmitting data with a delay fixed to the specified equalization delay. The above specification ensures that all the ONUs have the round-trip delay time of 20 km for the upstream data transmission.
In JP-A No. 2007-036920, there is a detailed description on the ranging in the PON system described above.
In such a PON burst receiving circuit, an offset is added to a threshold for determining “1” or “0”, in order to guard against white noise (hereinafter referred to as noise) occurring in a no signal timeslot, called a ranging window that is used for the ranging. Because the offset is added to the threshold that would have been in the middle of the peak and bottom values of the received signal, the threshold approaches the peak value by the amount of the offset. Hence, the probability of misidentifying “1” as “0” is higher than when the offset is not used, and the inclination of the burst reception characteristics is steep. Meanwhile, ITU-T Recommendation G.984.3 specifies Forward Error Correction (FEC) technology that can correct an error rate of 1e-4 to an equivalent of 1e-12, using a Reed-Solomon code. Larger coding gain can be obtained as the inclination of the PON burst reception characteristics is moderate.